1. Field of the Invention
The invention relates to devices for immobilising a piston in its cylinder and more particularly to the locking of a hydraulic jack at any point of its stroke.
2. Description of the Prior Art
There are known certain cases of the use of hydraulic jacks where the latter must maintain a considerable permanent stroke throughout an operation. This is the case, for example, of coupling devices between barges and tug-boats for navigation on the high seas. These two types of ships include special arrangements to ensure coupling by reason of the extremely high forces to which they will be subjected by bad weather. Thus, for example, the tug may be equipped at the front with oblique prismatic surfaces which become engaged under the thrust of the propellors in a cavity of conjugate shape situated at the rear of the barge, in the manner of a pyramidal wedge. To effect an absolutely rigid coupling at the end of insertion, the clamping force is ensured by a powerful hydraulic jack whose body is fast to the tug and whose piston rod end is fastened to the barge. This force must be maintained permanently throughout the duration of the journey. At the end of the operation, the force necessary to uncouple the barge and the tug will be higher by reason of the wedging forces created by the obliquity of the contact surfaces.
The maintenance of the clamping force during the period of coupling between the barge and the tug is indispensable and more particularly in bad weather. It was hence indispensable to effect the locking of the rod of the jack at the end of the operation of coupling the two ships. To this end, it is known that there exist already mechanical locking devices, among others that in which the end of the rod of the jack includes a screw element with interrupted threads engageable in a nut with an identical threading axially fast to the body of the jack, this engagement being made possible by placing face to face the smooth sectors of one of the parts with the threaded sectors of the other, the rigid linkage becoming positive by angularly limited rotation of the nut, which rotation is obtained by a hydraulic, pneumatic or electric motor.
In mechanical locking devices, the means for immobilising the rod are positioned so as to act as soon as the hydraulic clamping is achieved and the jack is no longer supplied. The rod of the jack is then more or less driven into its cylinder by reason of the variations in the relative positions of the barge and of the tug on each enclosure and especially because the barges can be different from one another. There must be hence taken into account these variations in position to establish the sizes of the locking members.
It has been proposed in U.S. Patent 3,150,571 of Frassetto et al to provide the end of a jack with a sleeve which is traversed by the rod of the jack with a frictional force such that this rod is immobilised. To unlock the jack, a fluid under pressure is introduced into the sleeve so as to produce its elastic expansion and thus to eliminate the friction. This device is suitable for jacks which only have to undergo a small number of operations, since wear and fatigue of the metal reduce the gripping force. Moreover irregularities in the rod cause differences in the braking force.